"Copy To selected" How do i copy only the material diffuse colour?
I think a small python script is the only way. This script only works for Blender Render.

import bpy

active_obj = bpy.context.object
active_obj_material = active_obj.active_material

for obj in bpy.context.selected_objects:
    if obj != active_obj:
        obj.active_material.diffuse_color = active_obj_material.diffuse_color


I use active_material so that objects with more than one material will use the material that is selected in the list, rather than forcing slot1 to always be used.